Process and assembly for non-destructive surface inspection

ABSTRACT

An optical system for detecting defects on a wafer that includes a device for producing a beam and directing the beam onto the wafer surface, producing an illuminated spot on the wafer&#39;s surface. The system further includes a detector detecting light, and a mirrored assembly having together with the detector an axis of symmetry about a line perpendicular to the wafer surface. The assembly is configured to receive scattered light from the surface, where the scattered light including a first scattered light part being scattered from the pattern. The assembly is further configured to reflect and focus rotationally symmetrically about the axis of symmetry the scattered light to the detector. The system further includes a device operating with the detector for facilitating detection of a scattered light other than the specified scattered light due to pattern.

FIELD OF THE INVENTION

The invention relates to the field of optical inspection. Morespecifically, the invention relates to the inspection of surfaces and inparticular to detecting defects in semiconductor patterned wafers.

BACKGROUND OF THE INVENTION

The detection of defects on the surface of semiconductor wafers due toimperfect production or the post-production adhesion process hasreceived considerable attention in the art. Generally, wafers fall intotwo main categories, “unstructured” (or “unpatterned”) and “patterned”.A patterned wafer has circuit patterns (“dies”) imprinted on it, whilean unstructured (unpatterned) wafer is still bare, i.e. with no circuitsimprinted on it as yet.

Generally speaking, numerous systems and methods have been developed tocope with the problem of defect detection and in particular, for thenon-destructive inspection of silicon wafers. A prior art system knownas the “Excite System” of Applied Materials includes a light beam sourceand an optical system that projects the beam onto the test object, aswell as means for detecting the reflected and/or scattered light. Thereis an additional assembly for moving the test object in a coordinatedtranslational and rotary movement, so that the light spot projectedthereon scans the whole surface along a spiral path. The detectedscattered light is analyzed in order to determine the sought defects.

The development of processes enabling the manufacture of wafer surfaceswith ever-finer structures, urged the development of inspection systemsfor the detection of ever more minute defects such as particlecontamination, polishing scratches, variations in the thickness ofcoatings, roughness, crystal defects on and below the surface, etc.Insofar as unstructured wafers are concerned, they are subjected to athorough searching examination for detecting said defects.

In the chip manufacturing process, it is common to monitor each stage inorder to recognize problems as early as possible and thus avoid unduewaste. When unstructured wafers are compared between two process stages,the types and amount of defects at some stage can be determined. Theinspected surface may be rough and metallized, and may therefore producea great deal of scattered light, or, it may be a film-coated surfacewith a small amount of defects and produce scattered light. Thus, theinspecting instrument should preferably have a wide dynamic range ofdetection to permit defect and particle detection of a wide variety ofsurfaces.

Laser scanners are particularly suitable for that purpose. Note thatpresently available laser scanners differ in the type of scanning theyuse, their optical configuration, and the manner in which the resultsare processed. For applications that require a high throughput andnearly 100% inspection of the whole wafer surface, two processes aremainly used. In the first, disclosed e.g. in U.S. Pat. No. 4,314,763 toSteigmeier & Knop, the illuminating beam and the collecting optics arestationary, and the test object is scanned spirally by means of acoordinated translational and rotary movement of the object itself. Inthe second process disclosed, e.g. in U.S. Pat. No. 4,378,159 toGalbraith, a rotating or vibrating mirror moves the illuminating beam inone direction linearly back and forth across the wafer, while the waferis simultaneously translated perpendicular thereto. In general, thefirst method is simpler and with homogenous accuracy, while the secondis faster.

Bearing all that in mind, attention is drawn to U.S. Pat. No. 6,271,916to Marxer et al. Briefly speaking, the Marxer patent discloses anassembly for non-destructive surface inspections. The system accordingto the '916 patent will now be briefly described with reference to FIGS.1A-B. Thus, the apparatus according to the Marxer patent includes alight beam that is directed by a beam deflector 113 and 131 towards thewafer's surface 135, preferably normal thereto. The wafer is moved by arotation motor 145 and a translation motor 149 according to thetechnique disclosed in the '159 patent. A circumferential ellipsoidalmirrored surface 127 is placed around the wafer, with its axiscoinciding with the surface normal, to collect scattered light fromdefects at the wafer surface at collection angles away from the surfacenormal. In some applications, a lens arrangement with its axiscoinciding with the surface normal is also used to collect the lightscattered by the surface and by any defects on it. The light scatteredby the mirror and lenses may be directed to the same or differentdetectors. Preferably, light scattered by the surface within a firstrange of collection angles from the axis is detected by a first detector121, and light scattered by the surface within a second range ofcollection angles from the axis is detected by a second detector 125(shown in FIG. 1B only). The two ranges of collection angles aredifferent, with one detector optimized to detect scattering from largedefects (mainly large particles) and the other detector optimized todetect light from small defects (particles). The content of the Marxerpatent is incorporated herein by reference.

The detectors according to the Marxer patent, detect practically onlylight scattered from defects, whereas reflected light (reflected from awell-polished surface) is out-guided in order not to interfere with thescattered light received by the detectors. This method of measuringdiffused light from defects only is called “dark field”.

The apparatus according to the Marxer patent offers a solutionapplicable, if at all, to the detection of defects on unstructuredwafers. However, the specified apparatus of the Marxer patent is notapplicable to the detection of defects on patterned wafers, because inthe case of patterned wafers, the detectors do not only receive lightscattered from defects, but also light scattered from the patterns.Considering that the intensity of the latter is much higher than that ofthe former, it would be very difficult and in fact practicallyinfeasible to determine whether the received light originates from adefect or from a fault-free pattern.

Die to die defect analysis is based upon a comparison (usually a on apixel to pixel bases or even a sub-pixel to sub-pixel bases) of pixelsoriginating from light scattered from the same spot on two distinctdies. Die to die comparison require hat substantially the sameillumination and collection conditions apply during the generation ofthe pixels. The Marxer patent does not enable die to die defect analysisas the wafer is rotated during the illumination of the wafer, and boththe illumination and collection paths constantly change as result fromthe wafers rotation. The problem is especially acute when the wafers arepatterned and when using dark field detectors to detect defects, as thedark field images are very dependent upon the direction of lightscattered from the rotating pattern. Accordingly, there is a need in theart to provide an apparatus that performs defect detection of patternedwafers.

There is another need in the art to provide an apparatus that performsdefect detection of both patterned and unpatterned wafers.

There is yet a further need to allow a compact optical inspectionapparatus that enables die to die defect analysis.

SUMMARY OF THE INVENTION

The invention provides for an optical system for detecting defects on awafer that includes at least one pattern; the system comprising:

-   -   a source of light to produce a beam;    -   optics directing the beam along a path onto the wafer, producing        an illuminated spot thereon;    -   at least one detector for detecting light;        an ellipsoidal mirrored surface, said mirrored surface and the        at least one detector having an axis of symmetry about a line        perpendicular to the wafer surface, said mirrored surface        defining an input aperture positioned proximate to the wafer        surface to receive scattered light therethrough from the        surface; said mirrored surface further defining an exit aperture        and being substantially rotationally symmetric about said axis        of symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through the input aperture to the at least one        detector;    -   said exit aperture being located opposite to the input aperture;        and    -   at least one filter located between said exit aperture and said        at least one detector and being configured to pass to said at        least one detector scattered light rays substantially other than        scattered light part being scattered from at least one of said        patterns.

The invention enables die to die defect analysis by implementing atleast one of the following measures: (i) illuminating the inspectedobject with a beam that is perpendicular to the surface of the inspectedobject, whereas the beam cross section is symmetrical and an array ofdetectors are positioned such as to collect scattered light beams; (ii)using a dove prism; rotating the optical detectors array such as tocompensate for the rotation of the wafer.

The invention further provides for an optical system for detectingdefects on a wafer that includes at least one pattern; the systemcomprising:

-   -   a source of light to produce a beam;    -   optics directing the beam along a path onto the wafer surface,        producing an illuminated spot thereon;    -   an array of detectors detecting light;    -   an ellipsoidal mirrored surface, said mirrored surface and the        array of detectors having an axis of symmetry about a line        perpendicular to the wafer, said mirrored surface defining an        input aperture positioned proximate to the test surface to        receive scattered light therethrough from the surface; said        mirrored surface further defining an exit aperture and being        substantially rotationally symmetric about said axis of        symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through the input aperture to the array of        detectors;    -   said exit aperture being located between said array of detectors        and said input aperture;    -   said array of detectors are adapted to detect scattered light        substantially other than scattered light part being scattered        from at least one of said patterns.

Still further, the invention provides for an optical system fordetecting defects on a wafer that includes at least one pattern; thesystem comprising:

-   -   a source of light to produce a beam;    -   optics directing the beam along a path onto the wafer, producing        an illuminated spot thereon;    -   at least one detector for detecting light;    -   an ellipsoidal mirrored surface, said mirrored surface and the        at least one detector having an axis of symmetry about a line        perpendicular to the wafer surface, said mirrored surface        defining an input aperture positioned proximate to the wafer        surface to receive scattered light therethrough from the        surface; said mirrored surface further defining an exit aperture        and being substantially rotationally symmetric about said axis        of symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through said input aperture to the at least one        detector;    -   said exit aperture being located opposite to the input aperture;    -   a Dove prism, having with the at least one detector an axis of        symmetry about a line perpendicular to the wafer's surface and        parallel to said Dove prism's base, said Dove prism being        rotated about said axis of symmetry, so as to rotate light        passing through said Dove prism at twice the angular velocity of        said Dove prism in the opposite direction about said axis of        symmetry; and    -   at least one filter located between said Dove prism and said at        least one detector and being configured to pass to said at least        one detector scattered light rays substantially other than        scattered light part being scattered from at least one of said        patterns.

Yet further, an optical system for detecting defects on a wafer thatincludes at least one pattern; the system comprising:

-   -   a source of light to produce a beam;    -   optics directing the beam along a path onto the wafer, producing        an illuminated spot thereon;        -   an array of detectors for detecting light;        -   an ellipsoidal mirrored surface, said mirrored surface and            the array of detectors having an axis of symmetry about a            line perpendicular to the wafer surface, said mirrored            surface defining an input aperture positioned proximate to            the wafer surface to receive scattered light therethrough            from the surface; said mirrored surface further defining an            exit aperture and being substantially rotationally symmetric            about said axis of symmetry, so that said mirrored surface            reflects and focuses rotationally symmetrically about said            axis of symmetry light that passes through said input            aperture to the array of detectors;        -   said exit aperture being located opposite to said input            aperture;        -   a Dove prism, said Dove prism and the array of detectors            having an axis of symmetry about a line perpendicular to the            wafer surface and parallel to said Dove prism's base, said            Dove prism being rotated about said axis of symmetry, so as            to rotate light passing through said Dove prism at twice the            angular velocity of said Dove prism in the opposite            direction about said axis of symmetry; said Dove prism            further being configured to pass to said array of detectors            scattered light rays substantially other than scattered            light part being scattered from at least one of said            patterns.

The invention provides for an optical system for detecting defects on awafer that includes at least one pattern; the system comprising:

-   -   a source of light to produce a beam;    -   means for directing the beam along a path onto the wafer,        producing an illuminated spot thereon;        -   at least one means for detecting light;            an ellipsoidal mirrored surface, said mirrored surface and            the at least one detecting means having an axis of symmetry            about a line perpendicular to the wafer surface, said            mirrored surface defining an input aperture positioned            proximate to the wafer surface to receive scattered light            therethrough from the surface; said mirrored surface further            defining an exit aperture and being substantially            rotationally symmetric about said axis of symmetry, so that            the mirrored surface reflects and focuses rotationally            symmetrically about said axis of symmetry light that passes            through the input aperture to the at least one detecting            means;    -   said exit aperture being located opposite to the input aperture;        and        -   at least one filter located between said exit aperture and            said at least one detecting means and being configured to            pass to said at least one detecting means scattered light            rays substantially other than scattered light part being            scattered from at least one of said patterns.

The invention further provides for an optical system for detectingdefects on a wafer that includes at least one pattern; the systemcomprising:

a source of light to produce a beam;

means for directing the beam along a path onto the wafer surface,producing an illuminated spot thereon;

-   -   -   an array of detecting means for detecting light;        -   an ellipsoidal mirrored surface, said mirrored surface and            the array of detecting means having an axis of symmetry            about a line perpendicular to the wafer, said mirrored            surface defining an input aperture positioned proximate to            the test surface to receive scattered light therethrough            from the surface; said mirrored surface further defining an            exit aperture and being substantially rotationally symmetric            about said axis of symmetry, so that the mirrored surface            reflects and focuses rotationally symmetrically about said            axis of symmetry light that passes through the input            aperture to the array of detecting means;

    -   said exit aperture being located between said array of detecting        means and said input aperture;        -   said array of detecting means are adapted to detect            scattered light substantially other than scattered light            part being scattered from at least one of said patterns.

Still further, the invention provides for an optical system fordetecting defects on a wafer, comprising:

-   -   a device for producing a beam and directing the beam onto the        wafer surface, producing an illuminated spot thereon;    -   at least one detector detecting light;    -   a mirrored assembly having together with the at least one        detector an axis of symmetry about a line perpendicular to the        wafer surface, said assembly is configured to receive scattered        light from the surface; said assembly further configured to        reflect and focus rotationally symmetrically about said axis of        symmetry the scattered light to the at least one detector; and    -   a device associated with said at least one detector for        facilitating detection of a scattered light substantially other        than scattered light part being scattered from at least one of        said patterns.

Yet further, the invention provides for an optical system for detectingdefects on a wafer, comprising:

-   -   a device for producing a beam and directing the beam onto the        wafer surface, producing an illuminated spot thereon;    -   at least one detector detecting light;    -   a mirrored assembly configured to receive scattered light from        the surface; said assembly further configured to reflect the        scattered light to the at least one detector; and    -   a device associated with said at least one detector for        facilitating detection of a scattered light substantially other        than scattered light part being scattered from at least one of        said patterns.

The invention provides for an optical method for detecting defects on awafer that includes at least one pattern; the method comprising:

-   -   providing a beam of light;    -   directing the beam along a path onto the wafer, producing an        illuminated spot thereon;    -   positioning an ellipsoidal mirrored surface and at least one        detector so that they have an axis of symmetry about a line        perpendicular to the wafer surface, said mirrored surface        defining an input aperture positioned proximate to the wafer        surface to receive scattered light therethrough from the        surface; said mirrored surface further defining an exit aperture        and being substantially rotationally symmetric about said axis        of symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through the input aperture to the at least one        detector; said exit aperture being located opposite to the input        aperture; and locating at least one filter between said exit        aperture and said at least one detector, configuring the at        least one filter to pass to said at least one detector scattered        light rays substantially other than scattered light part being        scattered from at least one of said patterns.

Still further, the invention provides for An optical method fordetecting defects on a wafer that includes at least one pattern; themethod comprising:

providing a beam of light;

-   -   directing the beam along a path onto the wafer surface,        producing an illuminated spot thereon;    -   positioning an ellipsoidal mirrored surface and an array of        detectors so that they have an axis of symmetry about a line        perpendicular to the wafer, said mirrored surface defining an        input aperture positioned proximate to the test surface to        receive scattered light therethrough from the surface; said        mirrored surface further defining an exit aperture and being        substantially rotationally symmetric about said axis of        symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through the input aperture to the array of        detectors; said exit aperture being located between said array        of detectors and said input aperture;    -   adapting said array of detectors to detect scattered light        substantially other than scattered light part being scattered        from at least one of said patterns.

Yet further, the invention provides for an optical method for detectingdefects on a wafer that includes at least one pattern; the methodcomprising:

-   -   providing a beam of light;    -   directing the beam along a path onto the wafer, producing an        illuminated spot thereon;    -   positioning an ellipsoidal mirrored surface and at least one        detector so that they have an axis of symmetry about a line        perpendicular to the wafer surface, said mirrored surface        defining an input aperture positioned proximate to the wafer        surface to receive scattered light therethrough from the        surface; said mirrored surface further defining an exit aperture        and being substantially rotationally symmetric about said axis        of symmetry, so that the mirrored surface reflects and focuses        rotationally symmetrically about said axis of symmetry light        that passes through said input aperture to the at least one        detector; said exit aperture being located opposite to the input        aperture;    -   positioning a Dove prism, so as to have with the at least one        detector an axis of symmetry about a line perpendicular to the        wafer's surface and parallel to said Dove prism's base; said        Dove prism is rotated about said axis of symmetry, so as to        rotate light passing through said Dove prism at twice the        angular velocity of said Dove prism in the opposite direction        about said axis of symmetry; and    -   locating at least one filter between said Dove prism and said at        least one detector and configuring the filter to pass to said at        least one detector scattered light rays substantially other than        scattered light part being scattered from at least one of said        patterns.

The invention provides for an optical method for detecting defects on awafer that includes at least one pattern; the method comprising:

-   -   providing a beam of light;    -   directing the beam along a path onto the wafer, producing an        illuminated spot thereon;        -   positioning an ellipsoidal mirrored surface and an array of            detectors so that they have an axis of symmetry about a line            perpendicular to the wafer surface, said mirrored surface            defining an input aperture positioned proximate to the wafer            surface to receive scattered light therethrough from the            surface; said mirrored surface further defining an exit            aperture and being substantially rotationally symmetric            about said axis of symmetry, so that said mirrored surface            reflects and focuses rotationally symmetrically about said            axis of symmetry light that passes through said input            aperture to the array of detectors; said exit aperture being            located opposite to said input aperture;    -   positioning a Dove prism so as to have with the array of        detectors an axis of symmetry about a line perpendicular to the        wafer surface and parallel to said Dove prism's base; said Dove        prism is rotated about said axis of symmetry, so as to rotate        light passing through said Dove prism at twice the angular        velocity of said Dove prism in the opposite direction about said        axis of symmetry; said Dove prism is further being configured to        pass to said array of detectors scattered light rays        substantially other than scattered light part being scattered        from at least one of said patterns.

The invention further provides for an optical method for detectingdefects on a wafer, comprising:

-   -   providing a device for producing a beam and directing the beam        onto the wafer surface so as to produce illuminated spot        thereon;    -   positioning a mirrored assembly and at least one detector so        that they have an axis of symmetry about a line perpendicular to        the wafer surface; configuring said assembly to receive        scattered light from the surface and further configuring said        assembly to reflect and focus rotationally symmetrically about        said axis of symmetry the scattered light to the at least one        detector; and    -   positioning a device associated with said at least one detector        for facilitating detection of a scattered light substantially        other than scattered light part being scattered from at least        one of said patterns.

Yet further, the invention provides for an optical method for detectingdefects on a wafer, comprising:

-   -   providing a device for producing a beam and directing the beam        onto the wafer surface so as to produce an illuminated spot        thereon;    -   positioning a mirrored assembly configured to receive scattered        light from the surface and further configuring said assembly to        reflect the scattered light to the at least one detector; and        positioning a device associated with said at least one detector        for facilitating detection of a scattered light substantially        other than scattered light part being scattered from at least        one of said patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1A-B illustrate schematically two embodiments of an apparatusaccording to the Marxer patent;

FIG. 2A shows a perspective view of an apparatus according to oneembodiment of the present invention;

FIG. 2B shows a perspective view of an apparatus according to anotherembodiment of the present invention;

FIG. 2C shows a perspective view of an apparatus according to stillanother embodiment of the present invention;

FIG. 3A shows a perspective view of a filter placed in the apparatusaccording to an embodiment of the present invention;

FIGS. 3B-C describe the rotation of the filter synchronized with thewafer's rotation, in accordance with an embodiment of the invention;

FIGS. 4A-B show a side view of a closed and open MEMS(Micro-Electro-Mechanical System), in accordance with an embodiment ofthe invention;

FIG. 4C shows a plan view if the MEMS illustrated in FIGS. 4A and 4B

FIG. 4D shows a filter comprised of a 2D matrix of MEMS, in accordancewith an embodiment of the invention;

FIGS. 5A-B illustrate two stages (pass/fail) of a liquid crystal basedfilter in accordance with an embodiment of the invention;

FIG. 5C shows a filter that is composed of 2D matrix liquid crystalcells, in accordance with an embodiment of the invention;

FIG. 6A shows a ring-shaped array of detectors, in accordance with anembodiment of the invention;

FIG. 6B shows concentric ring-shaped arrays of detectors, in accordancewith an embodiment of the invention;

FIG. 7A shows a perspective view of an array of detectors made of CCDs,in accordance with an embodiment of the invention;

FIGS. 7B-C show respective opening and closing of CCDS according towafer rotation, in accordance with the embodiment of FIG. 7A

FIG. 8A illustrates the principle of operation of a Dove prism;

FIG. 8B further exemplifies the rotation of an image due to rotation ofa Dove prism; and

FIG. 8C shows a perspective view of an apparatus according to stillanother embodiment of the present invention, using a Dove prism betweenmirror to detector.

FIG. 8D illustrates the dark-field light beam in the system, showing thepath through the optical system of the scattered rays due to pattern anddefects.

DETAILED DESCRIPTION OF THE INVENTION

It should be noted that in the context of the invention, the term“defect” should be construed in a broad manner including but not limitedto particle contamination, polishing scratches, variations in thethickness of coatings, roughness, crystal defects on and below thesurface etc.

A beam of light that impinges on the surface of a patterned waferproduces a reflected beam and multiple scattered rays due to pattern anddue to (possible) defects. The distribution of scattered rays due topattern is distributed substantially different than the distribution ofscattered ray due to defects. Thus, knowing in advance the distributionof scattered rays due to pattern, enables one to locate a set ofdetectors at places where the distribution of scattered rays due topattern is zero, or at least minimal, or alternatively block the lightrays at places where the distribution of rays due to pattern issignificant, and by this to detect mainly scattered rays due to defects.This method can be used to verify the presence of defects on a patternedwafer, as will be explained in detail below.

FIG. 2A shows apparatus 101 according to an embodiment of the presentinvention. The apparatus differs from that described in U.S. Pat. No.6,271,916 B1, in that it is operable to detect defects of patternedwafers. Apparatus 101 includes filter 310 that is disposed between anellipsoidal mirror 127 and a detector 121. The added filter is designedto block the path of scattered light due to the pattern, butsimultaneously let the scattered light due to a defect pass through, aswill be explained in greater detail below.

FIG. 2B shows apparatus 102 in accordance with another embodiment of theinvention. Apparatus 102 is similar to apparatus 101, but has multipledetectors, such as detector array 320, instead of filter 310.

FIG. 2C shows apparatus 103 in accordance with a further embodiment ofthe invention. Apparatus 103 is similar to apparatus 101 but hasadditional multiple detectors, such as detector array 320 locatedupstream in the paths of scattered and reflected light beams in relationto filter 310. The addition of these detectors enables to geometricallyselect the pattern of scattered light-rays to be detected, whichimproves the system's performance.

Note that although the light beam that impinges the wafer surface in theexamples of FIGS. 2A and 2B is perpendicular to the surface, theinvention is by no means bound by these specific embodiments, asexemplified in FIG. 2C. An exemplary embodiment where the light impingesthe surface in a non-perpendicular fashion is described with referenceto FIGS. 8C-D, below.

Further note that whereas for the convenience of explanation thedescription below concerns mainly a filter, it likewise applies to anarray of filters.

It also should be noted that the invention is by no means bound by thisspecific embodiment. Thus, for example, in accordance with a modifiedembodiment, any of the previous embodiments can be modified, e.g. toinclude a first optical means that collimate the scattered light to befiltered and second optical means to focus the filtered beam beforeimpinging on the detector. The first optical means can be used also formatching the diameter of the collimated beam to the diameter of thefilter 310 or to the diameter of the detector array 320. Both opticalmeans can be used also for fine-tuning the solid angle of the beam oflight impinging on the filter and/or the detector. In still anothermodified embodiment a lens assembly is disposed between the inputaperture and the exit aperture of ellipsoidal mirrored surface 127 tocollect light reflected from the wafer surface and passing through themirrored surface. The reflected light is then guided away from thefilter to be blocked or used for further detection.

Generally speaking, when the wafer rotates, different dies on the waferare exposed to the illumination spot for inspection purposes. Since alldies in the wafer have the same orientation, it readily arises that asthe wafer rotates, the inspected die's pattern is oriented at adifferent angle for each rotation. Accordingly, by the embodiments ofFIGS. 2A-2C, whichever the case may be, the filter 310 and/or detectors320 should be rotated or reconfigured in synchronization with that ofthe wafer rotation such that the filter will substantially block thescattered light due to the pattern and, by the same token, the detectorswill be substantially blocked from detection of the scattered light dueto the pattern. In contrast, the filter should substantially passscattered light due to defect(s) and by the same token, the detectorsshould substantially detect light due to defect(s). By “substantially”it is meant that not all scattered light due to defect(s) is passed ordetected, which the case may be. To this end, a controller 150 isutilized, as will be explained in greater detail below.

There are several ways to realize the above described process, as willnow be explained with reference to FIG. 3. and onwards. As shown in FIG.3A, apparatus 101 has filter 310 that includes disk 401, which is opaqueto light. In the disk 401 there are apertures 403 at pre-definedlocations such that scattered light due to defects can pass through saidfilter only through said apertures. A controller 150 that is coupled toboth the disk 401 and rotation motor 145 is configured to rotate thedisk about the axis of symmetry SR in a synchronized fashion with saidmotor, so as to let the light scattered from the defects pass throughsaid apertures and be detected by the detector 121. The scattered light(or major portion thereof) due to the pattern, impinges on the opaquesectors 401 and is blocked thereby, and consequently, will not reachdetector 121 and obviously will not be detected. Thus, the filterfunctions as a rigid mask to substantially block all scattered lightrays due to the pattern in accordance with the (same) rotationalmovement of all the detectors.

It should be noted that, as known per se, the shape of the filter (e.g.in the form of disk), and in particular the pattern of apertures such as403, is tailored to fit the pattern of the surface. Such filters areknown in the art. Such a disk may be either designed e.g. in accordancewith actual measurements of the distribution of light reflection due topattern, or as a consequence of a mathematical model describing thereflection and scattering pattern from a specific wafer.

Further note that a filter bank can be prepared in advance for a varietyof requirements. A filter assembly composed of several filters (e.g. inthe form of a large disk that contains several filters along itsperimeter) can be used as filter 403. For each type of patterned wafer,the most suitable filter on the filter assembly is chosen in accordancewith actual measurements of the distribution of light reflection due topattern received at the detector after being reflected by a each filteravailable on the assembly and selecting the most appropriate one.

The filter ensures that the scattered light rays that reach thedetectors are mainly or wholly due to defects. The rotation of thefilter is exemplified in FIGS. 3B-C.

Those versed in the art will readily appreciate that the invention isnot bound by the use of a disk with discrete apertures and particularlynot to the disk described in FIGS. 3A-C.

Another non-limiting realization of a filter is theMicro-Electro-Mechanical System (MEMS) 2D array technique shown in FIGS.4A-D. The MEMS array-of-shutters functions exactly as a filter describedabove with reference to FIGS. 3A-C, except for the fact that therotation is done electronically Thus, FIG. 4A schematically describesthe structure of a rectangular MEMS shutter. It is noted that othershaped MEMS arrays, such as circular MEMS arrays may be utilized. MEMSarrays are known in the art. FIGS. 4A-D illustrate a typical MEMS array.The etching of a thick silicon wafer bedding 500 produces a thin sheetof silicon 505 that is partially surrounded by a narrow spacing 507 andis also connected to a thick sheet of silicon 503. The narrow spacing507 is generated by etching the whole silicon thick layer. Therefore,the thin sheet 505 is able to bend to some degree. The addition ofelectrodes 509 enables to realize the bending of the thin silicon sheet(FIG. 4B) or aligning it back (FIG. 4A), thus giving rise to an openingor closing of the shutter. The MEMS technique enables to produce a 2Darray of, say, hundreds of shutters, a number large enough to make aneffective filter for the purpose of the present invention, as shownschematically in FIG. 4D.

In order to block scattered light due to a die pattern, some of theshutters should be closed in a pattern that fits the pattern of saidscattered light, i.e. scattered light due to the pattern should beblocked (by impinging on closed shutters) and scattered light due todefects should pass through open shutters, similar to the configurationdescribed above, with reference to FIG. 3A. To this end, a controller ofthe kind described above should be employed to synchronize between thewafer rotation and the opening/closing of the MEMS shutters.

Those versed in the art will readily appreciate that the invention isnot bound by the use of an array of shutters and particularly not to theMEMS array described in FIGS. 4A-D.

Another non-limiting realization of a filter is the Liquid CrystalDisplay (LCD) technique, presented in FIGS. 5A-C. As is generally knownper se, LCD unit (or cell) is a device having a first polarizer and asecond polarizer, both defining a space in which a liquid crystal isplaced. A liquid crystal is fluid like a regular liquid but isanisotropic in its optical and electromagnetic characteristics like asolid, due to the high orientational order of the liquid crystalmolecules (620 in FIG. 5A).

When plane-polarized light passes through a liquid crystal, themolecules of the liquid crystal rotate the plane of polarization of thelight. Light that passes through the first polarizer 640 is polarized.The polarized light passes then through the liquid crystal, whichrotates the plane of polarization of the passing light. The secondpolarizer 660 is placed at the exit of the liquid crystal. Theorientation of the second polarizer is chosen to be parallel to thepolarization of the light emanating from the liquid crystal (e.g.perpendicular to first polarizer, but in no case parallel to it). Thus,the liquid crystal guides the polarized light from the first polarizerso that the light may be transmitted through the second polarizer.

When an external voltage 610 is applied across a liquid crystal cell,the liquid crystal molecules (630 in FIG. 5B) are aligned in parallel tothe electric field that is induced by the external voltage, and cannotrotate the plane of polarization of the passing light anymore. Thus,light cannot get out of the device any more. Therefore, applying avoltage on the LCD based device 601 in FIG. 5B blocks the light inanalogy to the functioning of the MEMS device. The LCD 2D array in FIG.5C functions similarly to the 2D MEMS array explained above, wherevoltage is activated (open) MEMS is functionally analog to anon-activated (open) LCD cell 600 and close MEMS is functionally analogto a voltage activated (close) LCD cell 601. The device uses acontroller of the kind specified above to synchronize between theopening/closing of LCD shutters and the wafer rotation.

Those versed in the art will readily appreciate that the invention isnot bound by the use of an array of shutters and particularly not to theliquid crystal array described in FIGS. 5A-C.

Note that the blocking of scattered light due to the die's pattern canbe realized also by using an array of detectors. The array of detectorsis adapted to detect scattered light substantially other than saidscattered light due to a pattern. By one embodiment, this is realized ina way that the detectors in the array are switched on or off via acontroller in a synchronized manner to the rotation of the wafer as inthe case of a filter described with reference to FIG. 2A above. By wayof another example, this may be realized by switching on all thedetectors but reading under the control of the controller only dataindicative of scattered light substantially other than said scatteredlight due to pattern

Note that each detector has its own light collection zone. The lightcollection zones of different detectors may vary in shape, in sizeand/or in their direction. The light collection zones of neighboringdetectors preferably partially overlap, so as to ensure coverage of thewhole detection area.

There may be many realizations that utilize an array of detectors. Therefollows a description of two non-limiting embodiments.

In accordance with a first realization described with reference to FIG.6A, detector array 320 includes a plurality of detectors collectivelydenoted 700 that are arranged in a ring shape. Such a device may includee.g. several tens of detectors, such as detectors 701 (e.g.Photo-multiplier Tubes (PMTs), Photodiodes, Avalanche Photodiodes),which is enough for obtaining considerable (but still rough) sensitivityto the rotation of dies.

Detector array 320 may also include multiple detectors arranged as a fewconcentric rings, as illustrated at FIG. 6B. All the ringed arrays ofdetectors are concentrically placed at one plane and oriented towardsthe same location. This configuration enables to add more detectors andimproves the configuration's sensitivity to the angular orientation ofthe die.

Those versed in the art will readily appreciate that the invention isnot bound by the use of ringed arrays of detectors and particularly notto the array of detectors described in FIGS. 6A or 6B.

Another realization is shown in FIGS. 7A-C. FIG. 7A illustratesapparatus 102 in which detector array 320 includes a CCD array. A CCDarray 800 is an array of light-sensitive elements 802, which are, infact, some small electronic capacitors that are charged by the electronsthat are generated by incident light. The array may be implemented by atleast one CCD chip, each CCD chip including multiple CCD detectingelements. Common CCD chips are composed of a large number of detectingelements, referred to also as pixels (e.g. 192*165, 512*512, 1024*1024or more). Thus, the use of a CCD array is advantageous as compared to anarray of regular detectors, in that the number of detectors (i.e. cells)is enormously higher than in the former realization. This makes the CCDarray much more accurate and sensitive to small angle rotations.

The main disadvantage of using a CCD array is the huge data ratedelivered as an output of the CCD. The sampling rate of dies on a waferis very high, typically, although not necessarily, about 10⁷samples/sec. Thus, the data rate that should be delivered from a CCD isabout N*10⁷ pixels/sec, where N is the number of CCD elements. Since atypical CCD has about 10⁴-10⁶ pixels, the expected output data rate isin the range of 10¹¹-10¹³ pixels/sec, which is well beyond the presenttechnology. An example for a fast CCD array is the PB-MV40 MegapixelCMOS Image Sensor of Photobit Company, which is capable of a digitaloutput of almost 10⁹ pixels/sec per second, at most one percent of theexpected rate.

A non-limiting solution to the problem of the data-processing bottleneckis by reading only a partial set of elements at each sampling (e.g. 802,not 804), since, anyway, not all of them are required for collectingscattered light from defects. Note that a CCD array composed of a largenumber of CCD chips, each chip having its own light collection zone, canbe partitioned so as to allow such a selection, by avoiding datacollection from chips that get scattered light due to pattern. Still,the amount of information is huge, rendering the data processingrelatively complicated.

Those versed in the art will readily appreciate that the invention isnot bound by the use of a CCD array and particularly not to the CCDarray described in FIGS. 7A-C.

Reverting now to FIG. 2C, it is possible to use both a filter and anarray of detectors. This combination adds a degree of freedom togeometrically select the pattern of scattered light-rays to be detected.This choice can improve the system's performance.

Another way to realize the apparatus according to the present invention,is by rotating the beam of scattered light at the outlet of the mirror127, instead of rotating a filter or detector-array for the samepurpose. This can be realized by using a Dove prism, whose principle ofoperation is schematically shown in FIG. 8A. A Dove prism is a prismwhose triangular head is truncated. When light rays enter an incidentalwall 901 of Dove prism 900, they get out of the other wall 902 in areversed order. When the Dove prism 900 is rotated around an axis DPparallel to its base, as is shown in FIG. 8B, the entering image isrotated too at twice the angular velocity of the prism. This isillustrated in FIG. 8B, where the image 910 is rotated by 180° withreference to the object 905, as compared to a 90° rotation of Dove prism900. Note that while rotating Dove prism 900 the field of view (e.g. animage 910 of a die on the wafer) is rotated too, but the-latter does notchange its location within the wafer's surface. Thus, using Dove prism900 obviates the need to rotate the filter in a controlled fashion or tocontrol the detector array as discussed above.

The use of a dove prism 900 allows performing die to die defectanalysis, as the dove prism rotation compensates for the wafersrotation. In other words, the dove prism provides substantially the sameillumination and collection conditions, regardless the rotation of thewafer. An image of a die can be stored to be later compared to an imageof another die or a to a golden die.

FIGS. 8C and 8D illustrate apparatus 104, in accordance with a preferredembodiment of the invention. Apparatus 104 differs from apparatus 101 ofFIG. 2A by setting Dove prism 900 disposed between the exit aperture ofellipsoidal mirror 127 and filter 310. In addition, lens assembly 930 isset for focusing the light beam onto the wafer surface and lens assembly940 for collimating the light rays scattered from the wafer surface dueto pattern and defects, while lens assembly 960 is set to focus thelight at the outlet of filter 310 on detector 121. Note that lensassembly 950 is needed only when filter 310 is used for apparatus 104,but not in the alternative where detector array 320 is used. Note thatcontroller 150 is required for rotating Dove prism 900 in a controlledfashion with the wafer rotation to compensate for the rotation of waferpattern, as explained above.

Thus, according to one embodiment of the present invention,schematically illustrated in FIGS. 8C and 8D, Dove prism 900 is rotatedat half the angular velocity of the wafer rotation in the oppositedirection. The image at the exit of the Dove prism is a static image ofa die, since the prism rotation compensates for the wafer rotation. Thefilter e.g. 310 or detectors array e.g. 320 as described above areplaced at the exit of the Dove prism.

FIG. 8C describes the path of the bright-field light beam in the system.A beam of light is guided by mirror 920 to lens 930 that focuses thebeam. The beam is guided through Dove prism 900 and the ellipsoidalmirror 127 onto the wafer surface. Lens 930 may be located also betweenDove prism 900 and ellipsoidal mirror 127, on the condition that it hasa ring shape, to allow the path of scattered rays upstream. Thereflected beam is guided back through ellipsoidal mirror 127 to lens 940that collimates the beam. The collimated beam passes through Dove prism900 and is guided away by mirror 970 to be blocked or for furtherdetection for other applications.

FIG. 8D describes the dark-field light beam in the system, showing thepath through the optical system of the scattered rays due to pattern anddefects. Lens assembly 950 and 960 form a relay assembly, which isintended to image the plane proximate to the wafer surface at a remoteplane where the detector or detector array are located. Note that thedetector (detectors array) are disposed relatively away from the surface(requiring thus the image at the remote plane) due to the relativelylarge size of the Dove prism. Relay assembly 950 and 960 enable todetect the image as if it were closer to the wafer. Lens assemblyfurther matches the diameter of the collimated beam to the diameter offilter 310. The scattered light rays are collimated by lens assembly 950to be guided through Dove prism 900. At the outlet of the Dove prism thecollimated beam passes through filter 310 and then is focused by lensassembly 960 to impinge onto detector 121.

Note that other embodiments of Dove Prism are applicable. For example,instead of using filter 310 and detector 121, detectors array 320 isused. In still another embodiment of the present invention, filter 310and detectors array 320 are used. The latter modifications aresubstantially similar to those described with reference 101-103 in FIGS.2A-C.

Further note that the embodiment of apparatus 104 has two advantagesover the embodiments of apparatuses 101-103 illustrated with referenceto FIGS. 2A-C. First, the image at the outlet of the Dove prism isstatic independent of the die's orientation. This means that the filteror the detector array gets practically the same distribution of lightrays for each die. On the contrary, for the embodiments 101-103 thefilter should be rotated in a controlled fashion (or the detector arrayoperated in a controlled fashion) so as to match the image rotation.Thus, in the embodiments 101-103 unavoidable errors occur due to thelimited resolution of the filter or of the detector array. The secondadvantage is that apparatus 104 is better adapted to use asymmetriclight source, i.e. light source directed at a first angle in relation tothe normal to the wafer surface (where the normal constitutes symmetryaxis SR). For all the embodiments of FIGS. 2A-C, the use of anasymmetric light source necessarily entails the rotation of the lightsource in a synchronized fashion with the wafer rotation, thusmaintaining the same orientation with the pattern of each die. Otherwisethe filter or detector array would receive a totally differentdistribution of scattered light due to the pattern for each die. Incontrast, in apparatus 104, as was explained above, the resulting imageat the filter or detector array is static, due to the Dove prism, andtherefore there is no need to rotate the light source.

Those versed in the art will readily appreciate that the invention isnot bound by the use of a rotating prism and particularly not to theDove prism described in FIGS. 8A-C.

The invention has been described with a certain degree of particularity.Those versed in the art will readily appreciate that the invention isnot bound by the particular configurations described with reference toFIGS. 2 to 8 above or by the specific apparatus disclosed in the Marxerpatent.

1. An optical system for detecting defects on a wafer that includes atleast one pattern; the system comprising: a source of light to produce abeam; optics directing the beam along a path onto the wafer, producingan illuminated spot thereon; at least one detector for detecting light;an ellipsoidal mirrored surface, said mirrored surface and the at leastone detector having an axis of symmetry about a line perpendicular tothe wafer surface, said mirrored surface defining an input aperturepositioned proximate to the wafer surface to receive scattered lighttherethrough from the wafer surface; said mirrored surface furtherdefining an exit aperture and being substantially rotationally symmetricabout said axis of symmetry, so that the mirrored surface reflects andfocuses rotationally symmetrically about said axis of symmetry lightthat passes through the input aperture to the at least one detector;said exit aperture being located opposite to the input aperture; and atleast one filter located between said exit aperture and said at leastone detector, and being configured to substantially block a portion ofthe scattered light corresponding to the at least one pattern of thewafer and to substantially pass to said at least one detector anotherportion of the scattered light substantially other than the scatteredlight corresponding to the at least one pattern of the wafer.
 2. Theoptical system of claim 1, wherein the at least one filter comprises adisk with apertures at pre-defined locations and being rotated, under acontrol of a controller, about said axis of symmetry; said controller isconfigured to rotate said filter so as to pass to the at least onedetector, through said apertures, scattered light rays substantiallyother than said scattered light part to be detected by the at least onedetector.